The present invention relates to a packing laminate comprising a carrier layer of a material imparting stiffness and at least one layer of liquid-tight material.
Edible oil or food products which contain edible oil are usually packed in lacquered sheet metal cans, glass bottles or blow moulded plastic bottles. These types of packages are comparatively expensive to manufacture and to handle. Accordingly it is desirable to provide a more rational and less cost-demanding packing container for edible oil products. A suitable type of package could be e.g. the parallelepipedic packing container of laminated material which at present is generally used for the packing of other liquid foodstuffs, e.g. milk and fruit juices. These packing containers are manufactured from a packing laminate, comprising layers of paper, plastics and aluminium foil which by means of folding and sealing is converted to a substantially parallelepipedic container body with corner lugs flattened, folded in and sealed to the container body. Attempts to utilize a packing container of this type for the packaging of edible oil were found to give unsatisfactory results, since the plastic layer of polyethylene which covers the inside of the packing laminate, that is to say the side facing the contents, is not suitable for prolonged contact with edible oil or products containing edible oil. As the described type of package manufactured from the packing laminate has a number of advantages from the point of view of filling as well as handling and cost, it has been tried to overcome the described disadvantages by applying a layer of oil-resistant plastic material to the inside of the laminate. Experiments were carried out with various different types of plastics and in particular the types were tried which are used for the manufacture of the blow moulded plastic bottles mentioned earlier. The type of plastics generally employed for this purpose is a copolymer of ethylene and partially neutralized acrylic acid (EAA) which in the manufacture of blow moulded plastic bottles has proved to be oil-resistant. However, when this type of plastics is applied by extrusion to the inside of a packing laminate, its properties deteriorate so that the material when it has been in contact with oil for a prolonged period results in inferior elongation, and cracks appear when the material is stressed, e.g. by folding. This phenomenon is known as stress-cracking and practical experiments have shown that the coefficient of elongation is reduced to below 10% of the normal coefficient of elongation. The difference between EAA in blown and extruded form is due apparently to the processing temperature which is substantially higher (100.degree.-150.degree. C. higher) on extrusion form. A thermal degradation of the plastic material takes place which contributes to increased brittleness. A further cause is to be found in the different degrees of orientation existing in the blown or extruded film respectively. Blown film is oriented during manufacture in longitudinal as well as in transverse direction which results in manifestly better elongation properties than is the case in an extruded film which is only oriented in its longitudinal direction. Consequently, especially folds in the packing container running transversely in relation to the direction or orientation, will invariably cause crack formation with accompanying leakage and absorption of edible oil in the paper layer of the packing laminate, which impairs the stiffness of the laminate and gives the packing container a less attractive appearance.
It is an object of the present invention to provide a packing laminate which is suitable for the manufacture of packing containers for edible oil or products which contain edible oil, and which is not subject to the disadvantages of the types of packing laminate known previously.
It is a further object of the present invention to provide a packing laminate where the inner layer consists of an extruded plastic layer of a type of plastics which has good oil resistance.
It is a further object of the present invention to provide a packing laminate where stress-cracking does not occur during or after the contact of the laminate with oil.
These and other objects have been achieved in accordance with the invention in that a packing laminate comprising a carrier layer of a material imparting stiffness and at least one layer of liquid-tight material has been given the characteristic that the layer of liquid-tight material is an extruded plastic layer which comprises two types of plastics mixed with one another, namely LLDPE (linear low-density polyethylene) and LDPE (low-density polyethylene.
By mixing LLDPE and LDPE an internal layer packing laminate is achieved, the coefficient of elongation of which is not appreciably affected during extrusion of the layer, which means that stress-cracking does not occur even on prolonged contact with edible oil. To assure a good adhesion between the inner layer consisting of LLDPE and LDPE and adjacent layers of usually aluminium foil, use is made of an intermediate adhesion layer which preferably consists of a partially neutralized EAA (ethylene acrylic acid), that is to ay a copolymer of ethylene and acrylic acid.